In some vehicle transmissions, a ratio varying unit (“variator”) is used to provide a continuous variation of transmission ratio rather than a series of predetermined ratios. These transmissions may be referred to as continuously variable transmissions, infinitely variable transmissions, toroidal transmissions, continuously variable transmissions of the full toroidal race-rolling traction type, or similar terminology. In such transmissions, the variator is coupled between the transmission input and the transmission output via gearing and one or more clutches. In the variator, torque is transmitted by the frictional engagement of variator disks and rollers separated by a traction fluid.
The variator torque is controlled by a hydraulic circuit, which includes hydraulic actuators (i.e., pistons) that apply an adjustable force to the rollers. The force applied by the hydraulic actuator is balanced by a reaction force resulting from the torques transmitted between the surfaces of the variator disks and the rollers. The end result is that in use, each roller moves and precesses to the location and tilt angle required to transmit a torque determined by the force applied by the hydraulic actuators. A difference in the forces applied to the rollers changes the rollers' tilt angle and thus, the variator ratio. A change in the rollers' tilt angle thus results not only in a net torque at the transmission output but could also result in a change in torque direction. The direction of the torque output determines whether the torque application is positive or negative.
Some continuously variable transmissions have multiple operating modes, wherein each operating mode covers a portion of the overall ratio spread of the transmission. Each operating mode is selectable by a clutch that is engaged by the application of hydraulic fluid pressure as commanded by the transmission control unit. A transition between two operating modes involves a synchronous shift, in which there is a momentary overlap between the off-going clutch and the on-coming clutch. This momentary overlap results in a fixed ratio in which power is transmitted independently of the variator.
Prior to a mode transition, the variator ratio moves toward the ratio limit for the off-going mode, but the force applied to the rollers by the hydraulic actuators must be reversed in order to accomplish the transition to the on-coming mode. For example, in a two-mode continuously variable transmission having a low mode and a high mode, a low mode clutch is engaged and a high mode clutch is disengaged when the transmission is operating in the low mode, and when the transmission is operating in the high mode, the high mode clutch is engaged and the low mode clutch is disengaged. In the low mode, torque is produced by hydraulic forces acting on the variator rollers in one direction, and in the high mode, torque is produced by hydraulic forces acting on the variator rollers in the opposite direction (as compared with the low mode operation). Thus, during a mode transition (e.g. from low to high or vice versa), the direction of force applied to the variator rollers by the hydraulic control circuit is reversed, but the clutches ensure continuous power transmission to the transmission output through the synchronous shift.